Ingot molds provided with a hot-top



Aug. 21, 1956 R. B. GORDON ET AL 2,759,230

INGOT MOLDS PROVIDED WITH A HOT-TOP 2 Sheets-Sheet 1 Filed Jan. 12, 1952 Effect of Ingoi size on Required Heoi Input for at least 80% Sound Metal Recovery.

INVENTORS Ezekiel F. Losco gad Robert 8 Gordon W ATTO RNEi lngof Weight- Lbs.

WITNESSES:

Aug. 21, 1956 R. B. GORDON z-rrm. 2,759,230

INGOT MOLDS PROVIDED WITH A HOT-TOP Filed Jan. 12, 1952 2 Sheets-Sheet 2 Recovery of sound ingot metal as affected by extraction of heat from steel and mold walls. I00- 0 50 Ingot 1* 25 Ingot MgO Stool Stainless Steel Stool Cost iron Stool Copper Stool Recoveryof sound lngot Metal-percent I60 260 360 460 soo 600 Temperature difference,C,between mid-length and bottom of mold woll l5minutes otter teeming.

O 5- F|g.4. 3 J: m 0 $5 5 I: I 65 E g' 2 1; 2 E O 2 l- O l I I l 2 4 e 8 IO l2 Length of horizontal Columnor Groins ot mid-length Cross section of ingot-Inches. WITNESSES: INVENTORS a. Ezekiel F. Losco W 3 gnd RobertB.G0rdon.

ATTOR EY United States Patent Ofice 2,759,230 Patented Aug. 21, 1956 INGOT MOLDS PROVIDED WITH A HOT-TOP Robert B. Gordon and Ezekiel F. Losco, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 12, 1952, Serial No. 266,134

2 Claims. (Cl. 22139) This invention relates to ingot molds and to processes for the casting of molten metal therein to produce ingots having a high proportion of recoverable sound metal.

In teeming molten metal into ingot molds, it is desirable that as great a proportion as possible of the resultant ingot be a metallurgically sound product. Recoverable metal comprises that portion from the bottom of the ingot up to the highest cross-section level free from flaws. Unsoundness in an ingot may comprise pipe, cavities that may be either macroscopic or microscopic, or loose intergranular structure present within the interior of the ingot. Furthermore, impurities in the metal teemed into the mold tend to concentrate toward the central axis of the ingot as well as rising upwardly. Excessive amounts of the impurities along the axis are not desirable. Due to these and other flaws, it is a necessary practice in the metals industry to crop the upper end of cast ingots since flaws and unsound structure are concentrated within the upper portions thereof.

Ordinarily, about of a carbon steel ingot is cropped, leaving a recovery of less than 90% of substantially sound steel. However, many metals have a higher volume of contraction during solidification than does carbon steel or a low alloy steel. As the coefl'icient of volume contraction increases, the problem of producing ingots with a high proportion of sound metal increases manyfold.

The refractory alloys mentioned in the article entitled Precipitation-hardened alloys for gas-turbine service on page 583 of the August 1947 issue of Transactions of the A. S. M. E. and the numerous alloys discussed in the article entitled Gas-turbine alloys, 10 years later on pages 503 to 511 of the October 1950 issue of Metal Progress have been found to be very difficult to cast into ingots having a high percentage of recoverable sound ingot. A 70% recovery of ingots of these alloys is considered quite good. A desideratum in the metallurgical art is to secure a consistently high recovery of over 70% of each heat cast as sound ingot of these refractory metals, and preferably a recovery of 80% or more.

These refractory alloys containing large amounts of alloying elements usually have a relatively high volume contraction on solidification, and the problems encountered in producing ingots therefrom with a reasonably high recovery of sound metal of the order of 70% or higher have taxed the skill and ability of those working in the art. Particularly in casting relatively large ingots, it has not been feasible to secure consistently a high percentage of recovery of sound ingots.

The object of this invention is to provide a mold suitable for casting high alloy content ingots wherein the extraction of heat from the bottom of the teemed ingot is so correlated to the extraction of heat from the sides of the mold that horizontal columnar grains do not extend to the axis of the mold whereby a high proportion of recoverable sound metal is consistently secured.

A further object of the invention is to provide a mold for casting a sound ingot wherein there is correlated the extraction of the heat from the metal teemed into the mold so that the metal solidifies without building horizontal columnar grains extending more than 90% of the distance from the ingot walls to the axis of the ingot and a supply molten metal is maintained at the top of the ingot to fill any cavities developing by solidification contraction along the axis of the ingot.

A still further object of the invention is to provide an ingot mold in which heat is applied at the top of the ingot by an are developed between two arcing members so that, after teeming, the molten metal at the top of the mold remains fluid until the main body of the ingot has solidified into a sound ingot and is free from contamination.

Another object of the invention is to provide a process for casting ingots whereby heat is extracted from the bottom and sides of the ingot mold in a predetermined ratio so that the ingot solidifies progressively from the bottom to the top without developing horizontal columnar grains exceeding 90% of the distance from the mold walls to the axis of the ingot and molten metal is present at the top of the ingot until all the metal to a height equal to approximately of the length of the ingot has solidified.

Other objects of the invention will, in part, be obvious and will in part appear hereinafter. For a better understanding of the nature and object of the invention, reference should be had to the following detailed description and drawing, in which:

Figure l is a vertical section of a mold contracted in accordance with the present invention;

Fig. 2 is a graph plotting the amount of heat to be applied to the top of a teemed ingot in proportion to the amount of metal teemed to recover at least 80% sound metal.

Fig. 3 is a chart plotting the recovery of sound ingot metal against the temperature difference between the midlength and bottom of a metal mold wall 15 minutes after teeming, using stools of various materials; and

Fig. 4 is a graph plotting the length of the horizontal columnar grains at the mid-length portion of the crosssection of an ingot for various mold wall thicknesses at the mid-length portion of the mold.

In accordance with our invention we have produced an ingot mold in which molten metals having high coefiicients of volume contraction in passing from the liquid to the solid state, may be cast in accordance with a selected procedure, and will result in ingots having an exceptionally high percentage of recoverable sound metal. In the ingot mold of this invention, we have been able to correlate the extraction of heat throughout the entire length of the teemed ingot so that solidification progresses from the bottom of the ingot upwardly without permitting horizontal columnar grains of ingot metal to be built up from the sides of the mold walls to more than of the distance to the axis. By such a solidification schedule, a large unrestricted axial space is maintained in the teemed ingot into which molten metal is supplied from a pool of molten metal in the ingot head, thereby avoiding excessive piping, microscopic or macroscopic shrinkage cavities, or loose intergranular structure. By a high recovery of ingot, we mean approximately 70% or more of the ingot from the bottom up is metallurgically sound.

Briefly, the ingot mold of this invention comprises:

1. A metal stool made of selected metal and of a size and construction to Withdraw heat at a predetermined rate from a teemed ingot;

2. A tubular metal shell comprising the mold proper seated on the stool, the shell having a Wall thickness within prescribed limits;

3. A ceramic hot top fitting on top of the tubular mold;

4. A layer of thermal insulation about the tubular mold and ceramic hot top; and

5. A heater member, closing the open end of the hot top, for. maintaining teemed metal in the hot top moltenfon a1 predeterminedl length of time, so that it will fill any axial cavity. that developsasthe metal. solidifies.

Moreparticular ly, the ingot. mold produced in accordance with. one invention. comprises a metal stool of a metal having a thermal conductivityof at least 0.075 calorie per square centimeter per centimeter per degree centigradeper second. If it isnot water cooled, the metalt stool must be at least /2. v inch thick, the minimum thickness. increasing with the size of the ingot so that ininoieventi is it thinner than: 10% of the maximum crosswise on transveisedimension of an ingot being cast in the: mold. If water-cooling is; applied to the stool, the thickness is not significant. We have employed stoolsof a thickness-of inch with waterpassing at ahigh velocity int contact: with: thebottom side of' the stool. However,

water-cooled: stools for large ingots are preferably of a substantiali thickness and provided with drilledchannelsfor the passage of coolingliquid. The metal stool is so cons-tructedand ofisuch a size that it has a very substantial heat capacity. as-compared to the amount of heat to be withdrawn: from theteemed metal being'solidified so that thereis-arZOOf" C. difference in the temperature between the midpoint and the bottom of a metal mold disposed on the stool. The metal-stoolsfor larger ingots, that is 500 pounds and larger, are preferably water cooled.

Disposedonthe metal stool isa tubular open-ended metallshell ormold with its axis-vertically disposed, 'the metal mold being placed in direct and closecontact therewith-iso -thatheat is withdrawn fromthe moldwall by the stool. least inch-thiclt', but not exceeding a thickness equal to'45%:of the. shortest-distance from the-interior surface of themold'to the vertical axisof the-mold'at any point.

At the upper end of the tubular metal mold, an openended ceramic hot top-is placed: The ceramic hot top is at least=20%- of'the length of the metal mold in order to holda-substantial" amount of molten steel for filling shrinkage cavities-that'developas theteemed metalsoldifies aga-inst the metal-stool and meta'l mold wall.

Both the tubularmetal mold andthe ceramic hot top are surrounded=by= anex'terior layer of heat insulating material, such as sand, to-reducethe-amount'of heat withdrawn from. themolten metal through the sides of i the.

mold;

Ontop of the ceramic hot top is placed a heater member to-provide additional heat to maintain the teemed metal in-the hot top in a molten state'to enable filling of contraction cavities asthey develop during the progressive solidificationof-the ingot; The heater'member preferably comprises a ceramic cap closing the end-of the hot top, the cap-having a downwardly facing cavity in the cap disposed-to-refiect heat-to the metal'in-the ceramic hot top. Within the cavity there is a suitable heating device producing radiant heat so disposed that the walls of the cavity will-reflect such heat-down'into the metalin the hottop.

Referring to Fig. l of the drawing, there is illustrated an ingotmold 10 constructed in accordance with the present:invention. The ingot mold comprises a metal stool 12 of a metal having a thermal conductivity of 0.075 caloiie per square centimeter per. centimeter per degree centigrate-per second. Suitable metals for'this purpose are cast iron, carbon steel, low alloy steels containing not over 4 or v of alloying'elements, copper, nickel, beryllium-copper alloys, cupro-nickel alloy having. 20% nickel, aluminum bronze, and combinations of two or more of these. For casting relatively large ingots, channels 14 for the passage of water orother cooling liquid are present in the stool 121' It may be desirable, but not necessary, to provide an opening 16 in the bottom of the stool, the opening being normally filled with a plug 18 of graphite,

The tubular metal mold has a wall at 4' cast iron or the like. The graphite plug 18 is placed at the center of the stool so that metal teemed into the mold will strike thereagainst and minimize melting of the stool on impact with the stream of molten metal and thereby introducing an undesirable or unwanted amount of other metal into the ingot.

As examples of suitable stools, we have employed slabs of copper 6 inches thick and 2 5 inches by 25 inches as stools for molding ingots of from to 2400 pounds-in weight. Similarly, slabs of steel and cast iron of a thickness of up to 6 inches and dimensions of from 2 0- inches by 20 inches to 25 inches by 25 inches have been employed; On-the other'hand', we employed with success a stool comprising a inch thick tube flattened out to provide about inch spacing and forced'water therethrough at a high velocity.

Upon the upper surface 20 of the stool, there is placed a tubular metal: mold 2'2 with its major axis 24 being. vertically disposedi The base end 26 of the metal mold is in contact withand closely fits the face 20 of the metal stool. The tubular metalmold 22 may be of any suitable shape. We have employed circular cylindrical molds, rectangular molds, slightly barreled cylinders, and fluted molds. Successful results were had with tapered tubular molds of circularcross-section that were smaller; atthe bottom thanthe top,as well as with moldsthat were of uniform cross-sectional area. If molds of variable cross-section are used; the direction of taper is preferably with: the big end up. The metal mold may bemadez of any suitable metal capable of standing up" unden the moltenimetal, for example, steel, cast'ironand' variousalloys. Since therequirements for thermal conductivityare not as critical for the mold wall as for the stool, stainless steel-may be empl'oyedfor this purpose.

Inpracticingthe invention, We have used metal molds having a considerable variation in the ratioof the mini murntransverse dimension at the largest cross-section to thelengthor vertical height of the metal mold. Examples" of. ratios that have been-usedare from 0.3 to 0.8. In one instance,-a mold with an extreme ratio'of 0.27- was made. use of! Wehave discovered, as will be pointed'out in-detail' hereinafter, that the mold'wall thickness plays an extremely important part in determining the extent ofbuildup: of horizontal columnar crystals fromthe walls of the mold towards the axis. It is critically important to prevent these crystals fromreaching the axis of the mold,xand,' in-fact, a substantial axial area should be free from any" horizontal columnar crystals if sound ingots are to b'e produced. The solidification of metal inthezaxial core of the mold m'ust'be controlled'so that it progresses generally upwardly. To accomplish this, wehave-found it to be critic-althat the mold'wall thickness-of themetal mold, ab'ove the'lowermost 20% of its height; besuch that'no' substantial portion atanypoint exceeds a'thick'ness equal-to 4-5 of-th'e shortest distance from the interior'surface ofthe' mold'at-that point'to the vertical axis-24 ofthe tubular-mold.

The wall thickness of the tubular metal molds need not be uniform. We-have'employedmany varieties of shapes and proportions for the mold wall thickness: Substantial sized ingots have been cast-in-the tubular metalmolds of a'zthickness-of 0.04 inch; and 1200 pound ingots have been cast in molds-whose walls were 0.25 in'chthick.

At the'upper end 28'of the tubular mold 22, thereis placed an open-endedceramie-hot top 30"havingan'interior surface 32 generally in continuation of the interior surface of the tubular'm'etal mold. Disposed about the exterior of the metal mold 22 an'd'th'e ceramic hot'top 30" is a layer ofheat'insu'latingmaterial 38"retained in place by a'shell'40i. The heatiinsulatingmaterial '33 may. be sand, magnesium oxide, finely dividd'ceramic materials and the like, such'.materialsbeing capable of withstandingtemperatures that-may bev present when metal is cast in the mold. A layer of sand at least 2 inches thick about the mold and hot top has given good results in practice. The ceramic refractory member 30, insulating material 38 and shell 40 terminate in an upper surface 34.

Placed upon the upper surface 34 is a heater member 41 closing the opening in the hot top 30. The heater member 41 comprises a shell 42 of a suitable heat resisting metal containing a refractory ceramic cap 44. The ceramic cap 44 has an opening 46 through which metal may be teemed into the mold and after teeming, a plug 48 of graphite, ceramic or other refractory is placed therein to prevent loss of heat through the opening. Through the sides of the member 41 are placed hollow, electrically insulating, refractory members 50, for instance, of sillimanite, which extend through lateral projections 52 at opposite sides thereof. Passing through the hollow refractory members 50 are two carbon rods 54 and 56 connected to a suitable source of electric current (not shown). In operation, electrical current is supplied to the carbon rods 54 and 56 and an are 58 is drawn therebetween. The ceramic cap 44 has a downwardly facing cavity 60 essentially axially aligned with the axis 24. The cavity is preferably a paraboloid of revolution having its axis along the axis 24. However, it may comprise a rounded surface departing slightly from a parabolic crosssection. The are 58 is preferably generated, by suitable positioning of the carbon rods 54 and 56, so that its center is also on the axis 24. Radiant energy from the arc is radiated both directly and by reflection from the walls of the cavity 60 to the upper surface 62 of metal 6 that has been teemed into the ingot and maintains most of the portion thereof in the hot top 30 melted. A sight hole 66 may be present in the ceramic cap 44 for observation.

There must be maintained a correlation between the heat-absorbing capacity of the metal stool 12, the thick-v ness of the tubular metal mold 22, and the amount of insulating material 38 applied therearound so that there is present Within minutes after teeming a temperature difference of at least 200 C. between the bottom 26 of the tubular metal mold and the midpoint of the metal mold, that is, halfway between the surfaces 26 and 28. The maintenance of this temperature difference is one of the critical factors required to produce a high percentage of recoverable sound ingot, particularly of alloys characterized by a high volume contraction during solidification.

In Fig. 2, there is plotted a curve A of the total heat input required from the heater member 41 for various weights of ingots cast in the mold to insure at least 80% sound metal recovery. The curve A is the result of numerous tests in which ingots as small as and pounds and as large as 2400 pounds were employed in determining the amount of heat required to be applied to insure this high percentage of sound metal recovery. The equation corresponding to the curve is heat input in B. t. u. equals 70,000 plus 120 times the ingot weight in pounds. It will be understood that if a 70% recovery of sound metal would be acceptable, the heat input may be slightly lower than as indicated in Fig. 2. The total heat input comprises both the amount required to preheat the hot top and ceramic cap to a temperature of above 1000" C. and the amount applied after teeming. We have found that from 20 to 30 kilowatt hours (1 kilowatt hour equals 3413 B. t. u.) will preheat the ce ramic hot top and refractory cap of a 1200 pound ingot mold to about 1400-1450 C. For preheating other size molds to the same temperature, the heat input will increase or decrease from this value as the square root of the ratio of the ingot weight to 1200 pounds. Thus, for a 9600 pound ingot, the preheating will require from to 85 kilowatt hours.

In case unusually long ingots are cast, that is, the ratio of transverse dimension to length is less than 0.3, a slight increase of heat input is recommended. Thus f o r an ingot of a 0.27 ratio we find approximately 10,000 B. t. u. more should be applied than for a 0.8 ratio.

Since there is a consumption of the electrode carbons by oxidation, this oxidation introduces substantial amounts of heat to the mold, and it should be taken into account.

We have found that in 1 /2 hours of arcing, approximately 60,000 E. t. u. are contributed by the combusion of 1%. inch diameter carbon electrodes, i. e., at a rate of 40,000 B. t. u. per hour of arcing.

The heat input to the ingot may be supplied in part or entirely by other means than with an are developed between the two carbon electrodes. Gas burners or resistance elements may be substituted therefor. In particular the hot top may be preheated with a gas burner.

In certain instances we have applied heat generating compositions on top of the teemed metal in the ingot mold, and thereby supplied a part of the required heat. These heat generating compositions comprise combustibles such as aluminum or magnesium powder and oxidizing agents, etc. The trade designates them as liquidizers.

It will be appreciated that the required purity of the cast ingot metal and the relative reactivity of some of its components may cause a preference for one heating means over another after teeming. We have found that the arcing structure shown in Fig. 1 of the drawing it suitable for use in casting ingots of compositions that are extremely sensitive to the atmospheres present. However, the casting of conventional types of steel may be carried out in the presence of a gas-heating device with complete success.

Referring to Fig. 3 of the drawing, the points plotted illustrate the recovery of sound metal based on the temperature difference between the middlelength and bottom of the mold Wall 15 minutes after teeming. In this figure the points plotted represent stools of various indicated materials, namely, magnesium oxide, stainless steel, cast iron and copper. It will be noted that the magnesium oxide and stainless steel stools did not produce satisfactory ingots because of their inadequate thermal conductivity while the cast iron and copper stools were satisfactory. The stainless steel stool was used in producing an extremely small ingot weighing 25 pounds. If the ingot were 50 pounds or larger, the temperature difference using a stainless steel stool would have been considerably less than in the case of the 25 pound ingot, that is, such difference would be less than 200 C.

Using the ingot mold construction of this invention, the schedule of solidification of the teemed metal should be so conducted that the horizontal columnar grains that buildup from the tubular shell 22 will end a substantial distance from the axis 24. Under these preferred conditions a substantial volume of molten metal will be present along the axis during most of the solidification time. We have found that if the columnar grains reach the axis, then gases escaping from the solidifying melt will be entrapped. Furthermore, shrinkage cavities, either macroscopic or microscopic, may develop along the major portion of the axis of the ingot as a result of such excessive horizontal grain growth. Furthermore, if the axial portion is obstructed by such horizontal columnar grains meeting, molten metal cannot flow from the hot top down along the axis to fill any spaces that may develop due to volume contraction as metal solidifies.

The thickness of the mold wall is a highly important factor in determining the length of the horizontal columnar grains. From studies of a great number of ingots cast in molds of various shapes and sizes, we have obtained the information plotted as curve B in Fig. 4 of the drawing. It will be noted from curve B that the length of the columnar grains is roughly twice that of the mold wall thickness. We have determined that, to secure sound ingots, the mold wall thickness should be less than 45% of the distance from the interior surface of the mold at any point to the axis opposite that point. Inasmuch as the bottom 20% of the height of the mold is not invalved to any ex ent in the columnar grain growth, the wall thickness of this portion is irnrnat'eri'al, and this lower 20% of the meld may exceed this limit, but the upper 80% or the master mold must be so i-cper'nenee' that no substantial portion of the mold wall exceeds this maximum thickness. In practice, we prefer to maintain the mold wall thickness considerably below this 45% upper limit, inasmuch as our experience is that better ingot structures are obtained with much thinner mold walls. The wall thickness of the metal mold should not be subject to' abrupt changes in thickness. A uniform tapering of the mold walls with the thickest part at the bottom is preferred if the mold Wallis of a' non-uniform thicknes's.

The following proce'dure has given good results in casting" an ingot in the meld as shown in Fig. 1 of the drawing. Initially, the heater rnen'iber 41 is placed on the hot top of the empty mold 10, and an electric arc is produced between the carbon' rods 54 and 56 in order to' preheat the ceramic cap 44' and the ceramic hot top 30'. For highly refractory alloys, the preheated temperature of the hot top and the cap 44 adjacent the cavity 60 should be at least 1000 C., and preferably at the melting point of the metal being cast. We have heated them to temperatures from 1400" C. to 1500 C. and higher with excellent results when casting refractory metals. When the mold has been so preheated, the molten metal being otherwise ready for teeming, the plug 48 is withdrawn'fromthe' opening 46 and the carbon rods 54 and 56 are separated, the flow of electrical current, of course, having been interrupted. The molten metal is then teemed into the mold to fill' the tubular shell 22 and most of the ceramic hot top 30.

The plug 48 is then replaced and the carbon rods 54 and 56 are brought into arcing position and electrical current isa'pplied to cause an arc to be drawn therebetween. A high heat inp'ut is required during the first 30 minutes after teeming of themetal. In casting 1200 pound 'ingots, we have secured good results when at least 50 B. t1 u. per pound of metal was applied during the first half hour, that is, at a rate of at'least 25 kilowatts for the firsthalf hour plus 20,000 E. t. u. from combustion of the carbon electrodes. As examples, we have successfully used the following heating schedules in casting 1200 pound ingots: applying. 25 kilowatts for the first hour and 20 kilowatts for thesecond'hour, the average sound billet metal recovery for 10 ingots was 83.8%; when 30 'kilowa'tts are applied for one hour and kilowa'tts for the second hour, 10 ingots with an average metal'rec'o'very'of 82.2% were secured; 13 ingots with an average recovery of 83.6% sound billet were secured with 35 kilowatts bein'g'applied for one hour and 20 kilowatts for'a second hour; 88.7% of sound billet was secured for a number of ingots in which currentwasapplied to the heater at the'rate of 35' kilowatts for one-quarter hour; 20 kilowatts for another quarter of an hour and 10 kilowatts for one hour; and, more than 20 ingots havingan average metal recovery of 85.3% were secured when the electrical current to thearc was supplied at therateof 35 kilowatts for one-quarter hour, 20 kilowatts for three-quarters of an hour and finally 10'kilowatts for one hour. A number'of ingots having a recovery of 90% of sound metal were produced'when the electric arc functionedto'pr'ovide 35 kilowatts for onehalfhour', kilowatts for one-half hour and 15 kilowatts for one hour. In'ea'ch instance the oxidation of the carbon electrodes supplied 40,000 B. t. u. per hour of arcing. In all th'ese precedin'g'l ours, the metal was teemed at temperatures of from'1450" C. to' 1550 C., the ceramic hot top was' prehe'atedito about 1400 C. priorto'teeming, and the'metal m'oldcomprised fii inch thick sheet steel with approximately three inches of' sand insulationplac'edaboutthe nioldand ceramic hot top. The alloy castin these tests l was" th"e"highly refractory alloy-disclosed-' in Patent 2, 5l9',406. If the" ceramic" hot tsp has bees preheated S0 that it is below the; inertia; temperature of the molten metal being cast in the mold, a higher heat inp'u't during the first half hour than the 50 B'. t. u. per pound r mean being teemed will be 'required to maintain" a substantial pool of highly fiuid metal in the hot top". I

In using the mold of this invention in the manner prescribed, the teemed metal progressively solidifies rapidly from the bottom of the mold upwardly and at a slower rate from the side walls of the metal mold 22 toward the axis. In approximately one hour, the major proportion of the metal within the tubular metal mold 212 is solidified except fora substantial column along the axis 24. Within two hours, practically all of the metal in the mold 22 will be solidified arid only a small pool of molten metal within the hot top remains fluid. At this time, the electric current to the carbon rods 54 and 56 may be interrupted and the metal permitted to solidify entirely.

As examples of ingots cast with the mold constructed in accordance with this invention are the following:

Example I A sheet steel mold one-quarter inch thick and of a tapered rectangular cross-section having a dimension of 14 inches by 14 inches at the top and 13 inches by 13 inches at the bottom and of a length of 18 inches was placed on a copper stool 25 inches by 25 inches by 6 inches thickness with water cooling channels being present therein. A fire clay hot top 7 inches in length was placed on top of the metal mold. A four-inch thickness of sand insulation was placed around the entire mold and on the hot top. The hot top was preheated to a temper'ature of 1400 C. and after teeming, heat was applied at the rate of kilowatts for one-'quarter hour, 20 kilowatts for three-quarters of an hour and 10 kilowatts for one hour, providing a total of 33% kilowatt hours. The combustion of the electrode carbons introduced 60,000 B. t. u. Within 15 minutes after teeming, the midlength of the metal mold was at a temperature 480 C. higher than the bottom of the mold. The amount of sound metal recovered ffom the ingot was 94%. The horizontal columnar grains extended less than 2 inches from the mold walls.

Example II An alloy comprising 22% cobalt, 18% chromium, 42% nickel, 0.7% manganese, 0.7% silicon, 2.1 titanium, 03% aluminum and balance essentially all iron yields about a recovery of sound, usable metal when teemed into conventional molds which yield over 90% recovery of carbon steel ingots. A 1200 pound heat of this alloy at 1550 C. was cast in a tapered A1 inch thick sheet steel mold having internal dimensions of 14 inches by 14 inches at the top and 13' inches by 13 inches at the bottom and 18 inches long and placed on a water cooled copper stool 25 inches by 25 inches by 6 inches and provided with a graphite plug. A fire clay hot top 7 inches long was used. From 4% inches to 5% inches of sand was applied about the mold and hot top. A total of 225,000 B. t. u. was applied to the mold top, heat being applied at the rate of 30 kilowatts during the first half hour after teeming. In 15 minutes after teeming, the midpoint of the metal mold was 480 C. hotter than the bottom of the metal mold'. Over of sound ingot metal was recovered. The horizontal columar grains were 3 /2 inches in length.

Exa mple III A tapered sheet steel mold of a uniform wall thickness of /2 inch, with a square internal area of 16.5 by 16.5 inches at the top and 15 by 15 inches at the bottom and 24 inches long was placed on a cast iron stool 25 inches by 25 inches and of a thickness of 6 inches. A ceramic hot' to'p'apprjoximately 6 incheshigh was placed at the'upper iid of the sheet flie'tal'riiold, and 4 inches of sand were placed around the entire metal mold and hot top. The hot top was preheated to a temperature of approximately 1400 C. by an electrical are as shown in Fig. 1 before the metal was teemed. Within 15 minutes after teeming into the mold 2400 pounds of molten metal at a temperature of 1550 C., the composition being similar to that set forth in Patent 2,519,406, the temperature at the midpoint of the mold was 215 C. higher than the temperature at the bottom of the mold at the point of contact with the cast iron stool. A total input of 335,000 B. t. u. was applied including preheating by applying 40 kilowatt hours, and heating after teeming at the rate of 35 kilowatts during the first half hour, for another half hour at a rate of 20 kilowatts and 10 kilowatts for the next hour and 70,000 B. t. u. were introduced by the combustion of carbon electrodes.

The percent of sound metal recovered from the ingot after solidification is 73%. A section through the ingot showed the horizontal columnar grains were relatively small. This example shows the critical nature of the heating requirement. To secure 80% ingot recovery, the curve of Figure 2 requires 358,000 B. t. u. to be applied. However, a 73% recovery is still quite acceptable.

Example IV A 305 pound ingot of the copper alloy disclosed in Patent 2,033,709 was cast in a mold made of A inch thick sheet steel, the mold being 20 inches long. The sheet steel mold had a uniform cross-section of 7% inches by 7% inches. A fire clay hot top 4 inches long was placed on the mold. The mold rested on a cast iron stool 12 inches by 12 inches by 5 inches thick. The sheet steel mold and hot top were insulated with slightly more than 2 inches of sand. A total of 110,000 B. t. u. was applied by resistance heating elements, the mold hot top being preheated to 1700 C., to the mold to keep the copper alloy molten in the hot top, heat for the first half hour after teeming being applied at the rate of kilowatts. In minutes after teeming, a temperature dilference of over 200 C. was present between the bottom of the mold and the midpoint of the mold. An ingot comprising 100% of sound copper alloy was obtained.

The mold of the present invention may be employed for the casting of ingots of numerous metals and alloys; thus carbon steel, low alloy steel, nickel, high nickel alloys, stainless steel, and highly refractory alloys of all types are examples of metals that may be cast therein with a high recovery of sound ingot.

It shall be understood that the detailed description and drawing are exemplary and not in limitation of the molds or the process disclosed herein.

We claim as our invention:

1. A mold for casting ingots comprising, in combination, a vertically-disposed, tubular open-ended metal mold having a height greater than its transverse dimensions and having a wall thickness of at least & inch, and at no point above the lowermost 20% having a substantial portion of the wall of a thickness exceeding 45% of the shortest distance from the interior surface of the mold at that point to the vertical axis of the mold, a metal stool closely fitting and closing the base opening of the mold, the stool being of a metal having a thermal conductivity of at least 0.075 calorie per square centimeter per centimeter per degree centigrade per second, the metal stool cooperating with the metal mold to withdraw heat from a ferrous metal ingot cast therein at a rate such that Within 15 minutes after teeming there is a difference of at least 200 C. between the base portion and the midsection of the tubular mold, an open-ended refractory ceramic hot top placed on top of the tubular metal mold and in axial alignment therewith, the hot top being at least 20% of the length of the metal mold, a substantial thickness of thermally insulating material applied about the entire exterior of the hot top and the tubular metal mold to reduce the flow of heat from them, the thermally insulating material being equivalent to a layer of sand of a thickness of at least two inches, and a heater member disposed over the upper end of the hot top, the heater member comprising a refractory ceramic cap completely closing the end of the hot top with a downwardly facing cavity and a heating device comprising two electrical conductors disposed to produce an arc therebetween so disposed in the cavity that heat from the arc is reflected by the walls of the cavity downwardly into the end of the hot top.

2. The mold of claim 1, wherein the cavity is substantially a parabolic surface of revolution having its axis substantially in line with the axis of the tubular metal mold and hot top.

References Cited in the file of this patent UNITEDSTATES PATENTS 94,170 Abel et al. Aug. 31, 1869 272,682 Hainsworth Feb. 20, 1883 384,976 Hemphill June 26, 1888 1,527,521 Lewandowski Feb. 24, 1925 1,634,999 Krause July 5, 1927 1,892,044 Eldred Dec. 27, 1932 1,936,280 Williams Nov. 21, 1933 2,248,628 Hopkins July 8, 1941 2,544,598 Kalina Mar. 6, 1951 FOREIGN PATENTS 278,032 Great Britain Sept. 29, 1927 OTHER REFERENCES Electric Arc Hot Topping, by Sullivan in The Iron Age, February 11, 1943, pages 56-60. 

1. A MOLD FOR CASTING INGOTS COMPRISING, IN COMBINATION, A VERTICALLY-DISPOSED, TUBULAR OPEN-ENDED METAL MOLD HAVING A HEIGHT GREATER THAN ITS TRANSVERSE DIMENSIONS AND HAVING A WALL THICKNESS OF AT LEAST 1/32 INCH, AND AT NO POINT ABOVE THE LOWERMOST 20% HAVING A SUBSTANTIAL PORTION OF THE WALL OF A THICKNESS EXCEEDING 45% OF THE SHORTEST DISTANCE FROM THE INTERIOR SURFACE OF THE MOLD AT THAT POINT TO THE VERTICAL AXIS OF THE MOLD, A METAL STOOL CLOSELY FITTING AND CLOSING THE BASE OPENING OF THE MOLD. THE STOOL BEING OF A METAL HAVING A THERMAL CONDUCTIVITY OF AT LEAST 0.075 CALORIE PER SQUARE CENTIMETER PE CENTIMETER PER DEGREE CENTIGRADE PER SECOND, THE METAL STOOL COOPERATING WITH THE METAL MOLD TO WITHDRAW HEAT FROM A FERROUS METAL INGOT CAST THEREIN AT A RATE SUCH THAT WITHIN 15 MINUTES AFTER TEEMING THERE IS A DIFFERENCE OF AT LEAST 200* C. BETWEEN THE BASE PORTION AND THE MIDSECTION OF THE TUBULAR MOLD, AN OPEN-ENDED REFRATORY CERAMIC HOT TOP PLACED ON TOP OF THE TUBULAR METAL MOLD AND IN AXIAL ALIGNMENT THEREWITH, THE HOT TOP BEING A LEAST 20% OF THE LENGTH OF THE METAL MOLD, A SUBSTANTIAL THICKNESS OF THERMALLY INSULATING MATERIAL APPLIED ABOUT THE ENTIRE EXTERIOR OF THE HOT TOP AND THE TUBULAR METAL MOLD TO REDUCE THE FLOW OF HEAT FROM THEM, THE THERMALLY INSULATING MATERIAL BEING EQUIVALENT TO A LAYER OF SAND OF A THICKNESS OF AT LEAST TWO INCHES, AND A HEATER MEMBER DISPOSED OVER THE UPPER END OF THE TOP, THE HEATER MEMBER COMPRISING A REFRACTORY CERAMIC CAP COMPLETELY CLOSING THE END OF THE HOT TOP WITH A DOWNWARDLY FACING CAVITY AND A HEATING DEVICE COMPRISING TWO ELECTRICAL CONDUCTORS DISPOSED TO PRODUCE AN ARC THEREBETWEEN SO DISPOSED IN THE CAVITY THAT HEAT FROM THE ARC IS REFLECTED BY THE WALLS OF THE CAVITY DOWNWARDLY INTO THE END OF THE HOT TOP. 